A conventional disc brake system serves to slow translational motion of vehicles by converting the kinetic energy of the vehicle motion into heat. In conventional disc brake systems, the brake rotor includes a hat connected to an axle and a disc connected to the hat, the disc having on each side thereof a cheek upon which is opposingly applied a frictional force by brake pads of a brake caliper connected to a non-rotating component of the motor vehicle, whereby rotation of the wheel is slowed or stopped.
Conventional brake rotors of disc brake systems have been engineered to be strong, durable with respect to frictional wear. Further, conventional brake rotors are generally constructed of materials which have a high heat capacity so as to enable efficient management heat generated during frictional slowing of the motor vehicle.
Material costs have always been an important design consideration, particularly as they relate to vehicle fuel mileage. This trend has become so important that automotive engineers routinely take into account how an engineering change may affect vehicle fuel mileage.
With respect to vehicle fuel mileage, generally lighter vehicles translate into higher fuel mileage vehicles. Conventional brake rotors are constructed of homogeneous grey iron, commonly also referred to as cast iron, being generally an alloy of carbon, silicon and iron, having a mass on the order of about twenty-five pounds each. Therefore, for example, a motor vehicle equipped with four wheel disc brakes would utilize four brake rotors, totaling about one-hundred pounds, and constituting a significant proportion of the gross vehicle weight. Additionally, conventional brake rotors have an affect on vehicle dynamics, wherein, for example, lighter brake rotors would, in general, allow engineers to use lighter suspension components, thereby further reducing the gross vehicle weight.
Exotic materials, such as titanium, have properties of hardness and heat capacity superior to grey iron found in conventional brake rotors, and are routinely used in certain applications, such as aerospace. However, the cost of exotic materials renders simple substitution in mass-market products, such as motor vehicles, prohibitive.
Inexpensive metals, such as aluminum and aluminum-ceramic composites, have been used in motor vehicle brake rotors, as for example disclosed in U.S. Pat. Nos. 5,183,632; 5,224,572 and 5,884,388. Aluminum rotors have been found to possess a heat capacity which would render suitability in that regard; however, aluminum is much softer than iron based materials. Even if some surface hardening techniques, such as heat treating, could create a surface hardened case of aluminum substrate, the resultant aluminum-based material would still be significantly softer than grey iron-based materials.
Another strategy to reduce brake rotor mass in the prior art is to create areas of distinct homogenous regions of the base metals. Known casting techniques make it possible to cast adjacent materials without an alloying therebetween. For example, U.S. Pat. No. 4,930,606 discloses a composite brake rotor having a hat section including a non-planar rim to which is cast a rotor discoid which provides the rotor cheeks, wherein the rotor discoid is bonded to the rim of the hat section. The disclosure indicates that these rotors have a very uniform distribution of stress, so they are particularly resistant to mechanical and thermal stresses.
Another technique that creates distinct homogeneous regions of metal in a casting is discussed in U.S. Patent Application Publication 2003/00209288, wherein a method of manufacturing clad components consisting of two layers of metals is disclosed. A metallic substrate (i.e., a metallic sheet such as steel) has metal beads firmly bonded to the surface thereof via necks in the beads. The metal substrate with beads is placed into a mold, and a second metal, in a molten state, is then poured into the mold on top of the first metal substrate and is allowed to cool, forming a firm interlocking structure between the two metals via the necks of the beads, which may include metallurgical bonding. In this approach, for example, a steel surfaced aluminum brake rotor may be provided which is operable to a temperature over 1,400 degrees F. according to dynamometer tests, as recounted in the disclosure.
Accordingly, what remains needed in the art is a composite brake rotor that meets acceptable wear resistance and heat capacity standards while reducing the mass of the brake rotor when compared to conventional grey iron brake rotors and avoiding the problems associated with different materials surface boundaries bonding.